1. Field of the Invention
The present invention relates to an ink jet printing apparatus that prints an image by ejecting ink from a print head, and a method for controlling the ink jet printing apparatus.
2. Description of the Related Art
Some ink jet printing schemes of printing an image on a print medium by ejecting ink through a nozzle in accordance with an image signal use an electrothermal conversion element (heater) or a piezo element as an ejection energy generating element that allow ink to be ejected through the nozzle. A thermal ink jet scheme using the electrothermal conversion element applies an electric pulse to the electrothermal conversion element, which then generates heat to bubble ink, with the resulting bubbling energy utilized to eject the ink through the ejection port formed at the leading end of the nozzle. In such a thermal ink jet scheme, a change in environmental temperature or a change in the temperature of the print head associated with a printing operation may change the viscosity of the ink or the size of ink bubbles generated on the electrothermal conversion element. In such a case, the amount of ink ejected (the volume of the ink) and thus image printing density may be changed.
One method for preventing such a change in image printing density is a temperature rise control method of suppressing a change in the temperature of the print head by raising the temperature of the print head before starting printing. If such a temperature rise control method is applied to an ink jet printing apparatus with a plurality of print heads, the temperature of each of the print heads is raised to a reference value. However, when the print heads have different temperature rise characteristics or different temperatures before temperature rise, the print heads may reach the reference temperature at different timings. If a print head having reached the reference temperature first stops being heated and waits for the other print heads to reach the reference temperature, the temperature of the print head having reached the reference temperature first may decrease below the reference temperature during the wait. In this case, the print head with the temperature thereof having decreased below the reference temperature needs to be heated. It thus becomes difficult to quickly set the temperature of all the print heads equal to or higher than the reference temperature and then to start printing.
Japanese Patent No. 3900849 describes an ink jet printing apparatus in which a first reference temperature and a second reference temperature higher than the first reference temperature are set so that the apparatus raises the temperature of the print head until the print head reaches the second reference temperature and so that the apparatus starts printing when the print head exceeds the first reference temperature. The first reference temperature is a temperature at which the ink is ejected normally.